Binomial Probability for One x Value

To compute the binomial probability for
one particular number of successes, use the binompdf
function.

TI-89 Home Screen

TI-89 Stats/List Editor

Notice the difference: When you’re in the catalog, the
calculator puts you into alpha mode, so you don’t press
[ALPHA] to select a letter. But when you’re in the
Stats/List editor, you do press [ALPHA] to select a
letter.

Keystrokes

[CATALOG] [F3] [plain(makesB] [▼]

[F5] [ALPHA(makesB]

Format

binompdf(n, p, x)

(dialog box)

Example 1: Larry’s batting average is .260. If he’s
at bat four times, what is the probability that he gets exactly two
hits?

Solution:

n = 4, p = 0.26, x = 2

Note: Some textbooks use r for number of successes, rather than x.

binompdf(4,.26,2) = 0.2221

Binomial Probability for a Range of x Values

To compute the binomial probability for a
range of numbers of successes from xlow to
xhigh, use the
binomcdf function.

TI-89 Home Screen

TI-89 Stats/List

Keystrokes

[CATALOG] [F3] [plain(makesB]

[F5] [ALPHA)makesC]

Format

binomcdf(n, p, xlow, xhigh)

(dialog box)

Example 2: Larry’s batting average is .260. If he’s at bat
six times, what is the probability that he gets two to four hits?

Solution:

n = 6, p = 0.26, 2 ≤ x ≤ 4

binomcdf(6,.26,2,4) = 0.4840

Example 3: Suppose 65% of the registered voters in Dryden are
Republicans. In a random sample of ten registered voters, what’s
the probability of fewer than six Republicans?

Solution: “Fewer than six” is
zero through five.

n = 10, p = 0.65, 0 ≤ x ≤ 5

binomcdf(10, .65, 0, 5) = 0.2485

There’s about one chance in four of getting fewer than six
Republicans in a random sample of ten registered voters.

Example 4: With the same data, what’s the probability of getting
eight or more Republicans?

Solution: “Eight or more” means eight to
ten since there are only ten trials.

binomcdf(10,.65,8,10) = 0.2616

Example 5: A fair die has a 1/6 chance of rolling a 2. In 24 rolls,
what's the probability of getting no more than three 2’s?